The subject matter disclosed herein relates generally to a distributed motor drive system and, more specifically, to a system and method for managing harmonic currents present on a direct current (DC) link in a distributed motor drive system.
Alternating current (AC) motors receive an AC voltage at the stator of the motor. The speed and torque of the motor are controlled by varying the amplitude and frequency of this AC voltage applied to the stator. In order to provide varying AC voltage waveforms, a motor controller rapidly switches solid state devices on and off at a predetermined switching frequency and, thereby, alternately connects or disconnects the terminals of the motor to a DC voltage. By varying the duration during each switching period for which the terminal of the motor is connected to the DC voltage, the magnitude of the output voltage is varied. The motor controller utilizes modulation techniques such as pulse width modulation (PWM) to control the switching and to synthesize waveforms having desired amplitudes and frequencies.
Industrial applications which utilize servo motors, such as a process line with multiple stations, a machining center, or an industrial robotic arm, often have multiple axes of control. Each axis requires a motor and a controller to regulate, for example, the speed, position, or torque of the motor. The motors are necessarily positioned along the process line or about the machine as needed to actuate a specific motion. The motor controllers are typically located within one or more enclosures at a common location. However, developments in the power electronic devices used to control the motor have reduced the size of the components. This reduction in size of the power electronic devices along with a desire to reduce the size of the control enclosures have led to placing at least a portion of the motor controller electronics on the motor itself.
Such integrated motor and motor controller systems have not been fully met without incurring various disadvantages. According to one such system, the entire motor controller has been mounted on the motor. However, even with the reduction in size of the power electronic components, including the converter, inverter, and DC bus capacitance on the motor still requires a considerable amount of space, especially as the current rating of the motor increase. Further, the heat generated by both the converter and inverter power electronics must be dissipated at the motor.
According to another system for integrating the motor and motor controller, only the inverter section of the motor controller is mounted on the motor. The rectifier section and DC bus capacitance remains in the control enclosure. Although this system reduces the space required on the motor and also reduces the amount of heat that must be dissipated at the motor, another drawback arises. The inverter section receives a DC voltage via a DC link cable from a DC bus output of the rectifier section in the control enclosure. Although a small amount of capacitance may be connected across the DC link at the inverter section, modulation of the solid state devices in the inverter produces harmonic currents at multiples of the inverter switching frequency, which are, subsequently, conducted on the DC link between the inverter section and the control enclosure.
Another disadvantage in such a system is that the DC link cable extending between the control enclosure and the inverter section can establish a resonant frequency as a function of the length of the DC link cable. If the length of the DC link cable is selected such that the resonant frequency is close to the switching frequency of the inverter, the harmonic current on the DC link may be amplified. To avoid amplification of the harmonic current specific lengths of the DC link cable may be required. Alternately, to compensate for the harmonic content the size of the conductor is increased or the current rating of the conductor is reduced. Each of these options introduces an undesirable cost or limitation in the system.